Saturday, November 1, 2014

Bridges have been a feature of urban design ever since King Nabopolassar spanned the Euphrates River with a causeway around 620 B.C, joining together the two halves of the city of Babylon and much later inspiring the title of a Rolling Stones album. That this innovation represented a major improvement over the ferry transportation that had formerly prevailed was evident to ancient observers such as the Greek historian Herodotus, who noted that "under the former kings, if a man wanted to pass from one of these halves to the other, he had to cross in a boat; which must, it seems to me, have been very troublesome."

If the transportation advantages were clear at the time, the design challenges of incorporating bridges into a dense urban fabric presented difficulties that have continued to the present day. Apart from engineering challenges, the primary contextual concern is that a bridge high enough to avoid obstructing the flow of maritime traffic will typically be higher than the city itself, with the result that approaches to the bridge, if they are to accommodate wheeled traffic, will need to extend deeply into the city. Long approaches, however, disrupt and divide the urban fabric, undermining the very connectivity that the bridge was intended to provide.

For a well-known example, consider the Brooklyn Bridge, which was built to a height sufficient to accommodate the masts of sailing ships that still plied the East River in the early 1880s, and which, like Nabopolassar's bridge, replaced ferry services. An engineering marvel, the bridge was nonetheless so massive that its approaches reached deep into the heart of Manhattan, overshadowing many blocks and requiring the demolition of others:

Built some years before Robert Moses was even born, the bridge represented the first instance of an elevated roadway carving a swathe through a built-up area of Manhattan and dividing parts of the city from each other. In the years since the bridge was built, access ramps from the FDR Drive have further expanded the initial scar, leaving a gap of 360 feet in the city's fabric with limited crossing points. Although the arch spaces under the approach were creatively rented out as storage space for wine merchants (the bricked-in warehouse spaces can still be seen today), the effect on the immediately surrounding neighborhood could hardly have been a great positive. The area sliced up by the approaches to the Brooklyn, Manhattan and Williamsburg Bridges became notorious as Manhattan's Lower East Side, and was some decades later subjected to some of the most intensive urban renewal in the city. In Europe, where watersheds tend to be smaller than those of North America, major rivers narrower and where many bridges had been built long before the advent of suspension or steel-frame technology, a much more complementary design has long prevailed. Rather than sending approaches deep into the urban fabric, European cities tend to raise masonry embankments directly against the river, allowing a bridge even of substantial height to discharge traffic directly onto riverfront streets. Bridges were also considered architectural works in their own right intended to be experienced on foot, and incorporated sidewalk lighting, statuary, benches and other pedestrian amenities.

Pont Neuf, Paris. Google Maps.

Bird's eye view of another Pont Neuf, in Toulouse, with its entry point flush with a

Running along these embankments at just above water level are often found quays, which formerly served the shipping trade but which today have been converted to car expressways or recreational areas for cyclists and pedestrians (a notable conversion from the former to the later has recently taken place in Paris).In some famous instances, the city itself extended out onto the bridge, turning transportation infrastructure into a bustling city street with shops and homes. Among the best known of these are the former London Bridge, the Ponte Vecchio and the Rialto Bridge in Venice:

Paris seems to have had several such bridges as well, but most had their houses torn down in the late 1700s when the spatial demands of wheeled traffic began to make themselves increasingly apparent in the larger cities of Europe. The Pont Notre-Dame, above, was scraped clean of its tall dwellings in 1782, and the centuries-old bridge itself was replaced in the 1850s. London Bridge's houses, apparently allowed onto the bridge as a means of producing rent to offset the cost of bridge construction in the medieval period, were removed in the late 1750s at great expense to improve the bridge's level of service. The trend toward retrofitting cities around the needs of wheeled traffic would steadily accelerate through the late 20th century.

Source: Old Urbanist.

Some North American cities have bridges in approximately the European fashion, particularly where the city is located on a bluff overlooking a river or where the river is relatively narrow. Chicago, Milwaukee and San Antonio, in particular, have numerous such bridges over their relatively narrow rivers, and Austin has partial embankments overlooking a riverside trail. Des Moines, also, has a series of very European-looking bridges. Even if geography requires a bridge to enter a city at height, however, that does not mean that integrating it into the city need be impossible.

For instance, even if the ground level cannot be raised to meet the bridge, buildings themselves may be constructed up to the bridge level. The photo at right shows this approach deployed along a Danube River bridge in Regensburg, Germany (actually, in this case, I believe the bridge may have been constructed to align with the second floors of existing apartments). With this method, similar to the built-upon bridges described above, the bridge adds a second linear dimension to the city rather than simply being a passive structure accessible only at its endpoints. Additionally, the long approaches themselves are demanded only by wheeled traffic. Where a bridge serves only foot traffic, it is possible to provide high clearance, even with masonry construction, and yet have little or no landward approach. This method was employed abundantly in the towns and cities of pre-modern China, such as Wuzhen, below, where although the bridge appears to rise very steeply, the grade is quite a bit less than in the standard staircase, and the climb less arduous:

The American approach, reflective of the era of Heroic Materialism in general, has typically been to see bridges as engineering projects first, architecture second, and an integrated part of the city third, if at all. Even where existing bridges with lengthy approaches have been converted to pedestrian use, long approaches are typically retained, or in some cases, even rebuilt. The Big Four Bridge in Louisville, Kentucky, a rail bridge over the Ohio River which had its long approaches removed in the late 1960s, leaving only the central span, was inexplicably rebuilt with approaches even though it was intended primarily for pedestrians (amuch simpler plan requiring no land acquisition which would have involved a ramp directly around the final bridge pier was apparently rejected). On the Kentucky side, pedestrians must ascend a massive, circuitous and over-engineered ramp to reach the bridge:

A switchback staircase leading directly to the bridge pier was present during construction for the convenience of workers according to Streetview imagery, but seems to have been removed now that the approach is complete!Nashville's downtown Shelby Street Bridge, which never had its approaches demolished prior to its pedestrianization in the early 2000s, took a more sensible approach of adding a steel staircase and elevator, thereby taking advantage of the tremendous spatial efficiencies of pedestrianism while allowing people with bicycles, strollers or in wheelchairs to reach the bridge:

Google Maps.

In a first step toward directly integrating the bridge roadway with the surrounding buildings, the bridge and elevator are attached by an elevated walkway to the office building at the left. It is difficult to overstate the effect pedestrian infrastructure like this contributes toward making the bridge feel like a place, rather than an obnoxious intrusion into the life of the city.Turning bridge design away from the Heroic Materialist model of bridge-building toward a more pedestrian and city-oriented perspective is a long-term process that appears to be underway with bridge conversions, but many positive changes can done incrementally. Providing pedestrians with shortcut access points to bridge approaches, linking the bridge surface directly to surrounding buildings and even considering construction of new buildings flush with or underneath the bridge, can all help turn bridges into more than simply impressive engineering feats.Related posts: Jarrett Walker has a similar take on urban viaducts here (h/t to commenter Marc), and of course these observations could also be applied to other forms of elevated infrastructure to greater or lesser degrees.